Method and device for measuring the colour of an object

Abstract

The present invention relates to a method for measuring the uniform diffuse reflectance R OBJ (λ) at least at one point on an object ( 30 ) using a device ( 10 ) comprising a means ( 11 ) capable of emitting color illuminants expressed in the form of luminous flux and an electronic color image sensor ( 12 ). The present invention also relates to a device ( 10 ) comprising a means ( 11 ) for emitting color illuminants expressed as luminous flux of colors and an electronic color image sensor ( 12 ), for measuring the uniform diffuse reflectance R OBJ (λ) at least at one point on an object ( 30 ) placed in a zone located opposite and substantially perpendicular to the said means ( 11 ) capable of emitting colors and located in the field of vision of the said electronic color image sensor ( 12 ) and being subjected to an external illuminant expressed as a constant and unknown external environmental luminous flux ( 40 ) denoted I ext (λ).

The invention claimed is: 
     
       1. A method for measuring the uniform diffuse reflectance R OBJ (λ) at least at one point of an object ( 30 ) by using a device ( 10 ) comprising a means ( 11 ) capable of emitting coloured illuminants expressed in the form of luminous flux and an electronic colour image sensor ( 12 ), characterized in that it comprises the following steps:
 placing of the said object ( 30 ) in a zone located opposite and substantially perpendicular to the said means ( 11 ) capable of emitting coloured illuminants in the form of luminous fluxes of colours and located in the field of vision of the said electronic colour image sensor ( 12 ), the said object ( 30 ) also being subjected to an external illuminant in the form of a constant and unknown surrounding external luminous flux ( 40 ) I ext (λ), where λ denotes the wavelength; emission by the said means ( 11 ) of a series of N illuminants S source (λ) i  (with N being a natural number greater than one, i varying from 1 to N and λ being the wavelength), S source (λ) i  being known as a function of the input parameters of the said means ( 11 ) capable of emitting luminous fluxes of colours, capture by the said electronic colour image sensor ( 12 ) of the luminous flux reflected at least at one point of the said object ( 30 ) and entering in the sensor, the said luminous flux being denoted as E capteur (λ) i , with N being a natural number strictly greater than two, i varying from 1 to N and λ being the wavelength; and obtaining of N equations “E i ”: E capteur (λ) i =R OBJ (λ)*(I ext (λ)+S source (λ) i ) due to the additive nature of the wave light and by definition of the uniform diffuse reflectance R OBJ (λ) at least at one point of the object ( 30 ); and 
 determination by the said device ( 10 ) of the two unknown continuous functions R OBJ (λ) and I ext (λ) by solving the system of N equations E i :
 by integrating each equation E i  on the intersection of the source and sensor spectra, by denoting x, y and z the sensitivities in the colorimetric base selected, each equation E i  then generating three “E i  integrated” equations:
   ∫ E   capteur (λ) i   *x (λ)* dλ=∫R   OBJ (λ)*( I   ext (λ)+ S   SOURCE (λ) i )* x (λ)* dλ 
 
   ∫ E   capteur (λ) i   *y (λ)* dλ=∫R   OBJ (λ)*( I   ext (λ)+ S   SOURCE (λ) i )* x (λ)* dλ 
 
   ∫ E   capteur (λ) i   *z (λ)* dλ=∫R   OBJ (λ)*( I   ext (λ)+ S   SOURCE (λ) i )* x (λ)* dλ 
 
 
 by calculating the numerical value corresponding to the left-hand side of the Ei integrated equations with the use of the output parameters of the digital image sensor; and 
 by expressing the two unknown continuous functions R OBJ (λ) and I ext (λ) with the use of a finite number of interpolation points (λ j , y j ) connected by at least one interpolation function s(λ) for maintaining the continuous nature of the said unknown continuous functions R OBJ (λ) and I ext (λ), the λ j  being wavelengths selected in the intersection of the source and sensor spectra and being input parameters of the method, chosen to minimize the number of interpolation points for a given precision; and 
 by finding the parameters y j  of the functions R OBJ (λ) and I ext (λ) that minimize the least squares system ∥A*X−B∥ 2  resulting from the E i  integrated equations. 
 
 
     
     
       2. A method according to  claim 1 , characterized in that it further includes a step of determining the value of the external illuminant I ext (λ). 
     
     
       3. A method according to  claim 1 , characterized in that it includes in addition, a step of transcription of the function R OBJ (λ) of uniform diffuse reflectance at least at one point of the object ( 30 ) into the CIE XYZ coordinates for a given illuminant. 
     
     
       4. A method according to  claim 1 , characterized in that the series of N illuminants is of the same order of magnitude as the number of interpolation points for determining the values of the uniform diffuse reflectance R OBJ (λ) at least at one point of the object ( 30 ) and of the external illuminant I ext (λ). 
     
     
       5. A method according to  claim 1 , characterized in that it includes a step of determining the values of the uniform diffuse reflectance R OBJ (λ) at least at one point of the object ( 30 ) and the external illuminant I ext (λ) in several spectral bands. 
     
     
       6. A method according to  claim 1 , characterised in that the said device ( 10 ) makes use of a screen for emitting flashes of colour and an electronic image sensor for sensing and capturing the light reflected by the target object. 
     
     
       7. A method according to  claim 1 , characterised in that the said device ( 10 ) is a camera unit or a camera with in built or removable flash. 
     
     
       8. A method according to  claim 1 , characterized in that the said device ( 10 ) implements waveguides for ensuring effective transiting of the emission and reception of flashes of colours. 
     
     
       9. A method according to  claim 1 , characterized in that it is implemented in order to take spectrometric photographs of objects and to make chromatic adjustments (balancing of whites) at will. 
     
     
       10. A method according to  claim 1 , characterized in that it is implemented in order to measure the colour of an element included in the following group: materials, solids, liquids, gases, paintings, tapestries, graphics, textiles, plastics, woods, metals, soils, minerals, plants and foods. 
     
     
       11. A method according to  claim 1 , characterized in that it is implemented for the measurement of colours for medical or cosmetic purposes on human beings and living organisms of at least one element included in the following group: skin, pimples, moles, hair, fur, makeup, and teeth. 
     
     
       12. A method according to  claim 1 , characterized in that it is implemented for the use of colour barcodes, of one or more dimensions. 
     
     
       13. A method according to  claim 1 , characterized in that it is implemented with a view to assisting people having colour blindness and/or who are blind. 
     
     
       14. A device ( 10 ) comprising the means ( 11 ) capable of emitting colour illuminants in the form of luminous flux of colours and an electronic colour image sensor ( 12 ), for measuring the uniform diffuse reflectance R OBJ (λ) at least at one point of an object ( 30 ) placed in a zone located opposite and substantially perpendicular to the said means ( 11 ) capable of emitting colours and located in the field of vision of the said electronic colour image sensor ( 12 ), and also being subjected to an external illuminant in the form of a constant and unknown surrounding external luminous flux ( 40 ) denoted as I ext (λ), characterized in that it comprises the means for:
 emitting a series of N illuminants S source (λ) i  (with N being a natural number strictly greater than two, i varying from 1 to N and λ being the wavelength), S source (λ) i  being known as a function of the input parameters of the said means ( 11 ) capable of emitting luminous fluxes of colours, capture by the said electronic colour image sensor ( 12 ) of the luminous flux reflected at least at one point of the said object ( 30 ) and entering in the sensor, the said luminous flux being denoted as E capteur (λ) i , with N being a natural number greater than one, i varying from 1 to N and λ being the wavelength; and obtaining of N equations (E i ): E capteur (λ) i =R OBJ (λ)*(I ext (λ)+S source (λ) i ) due to the additive nature of the wave light and by definition of the uniform diffuse reflectance R OBJ (λ) at least at one point of the object ( 30 ); and 
 determining the two unknown continuous functions R OBJ (λ) and I ext (λ) by solving the system of N equations E 1 :
 by integrating each equation E i  on the intersection of the source and sensor spectra, by denoting x, y and z the sensitivities in the colorimetric base selected, each equation E i  then generating three “E i  integrated” equations:
   ∫ E   capteur (λ) i   *x (λ)* dλ=∫R   OBJ (λ)*( I   ext (λ)+ S   SOURCE (λ) i )* x (λ)* dλ 
 
   ∫ E   capteur (λ) i   *y (λ)* dλ=∫R   OBJ (λ)*( I   ext (λ)+ S   SOURCE (λ) i )* x (λ)* dλ 
 
   ∫ E   capteur (λ) i   *z (λ)* dλ=∫R   OBJ (λ)*( I   ext (λ)+ S   SOURCE (λ) i )* x (λ)* dλ 
 
 
 by calculating the numerical value corresponding to the left-hand side of the Ei integrated equations with the use of the output parameters of the digital image sensor; and 
 by expressing the two unknown continuous functions R OBJ (λ) and I ext (λ) with the use of a finite number of interpolation points (λ j , y j ) connected by at least one interpolation function s(λ) for maintaining the continuous nature of the said unknown continuous functions R OBJ (λ) and I ext (λ), the λ j  being selected wavelengths in the intersection of the source and sensor spectra and being input parameters of the method, chosen for minimizing the number of interpolation points for a given precision; and 
 by finding the parameters y j  of the curves R OBJ (λ) and I ext (λ) that minimize the least squares system ∥−A*−X−B∥ 2  resulting from the E i  integrated equations. 
 
 
     
     
       15. A method according to  claim 2 , characterized in that the series of N illuminants is of the same order of magnitude as the number of interpolation points for determining the values of the uniform diffuse reflectance R OBJ (λ) at least at one point of the object ( 30 ) and of the external illuminant I ext (λ). 
     
     
       16. A method according to  claim 2 , characterized in that it includes a step of determining the values of the uniform diffuse reflectance R OBJ (λ) at least at one point of the object ( 30 ) and the external illuminant I ext (λ) in several spectral bands.